Silicon dioxide fibers with high silicon dioxide content have been known for a long time. It is possible, for instance, to produce fibers from pure quartz; however, this requires very high temperatures. Such a process is highly energy-intensive; moreover, it requires elaborate equipment because of the high melting point of quartz, so that melt-spun silica fibers end up being very costly to produce.
A large part of the sodium oxide and oxides (other than silicon dioxide) can be removed from glass fibers by acid leaching. But the mechanical properties of fibers treated in such manner are not entirely satisfactory, particularly with respect to their resistance to very high temperatures.
Efforts have been made to enhance the characteristics of a great variety of inorganic fibers partially or entirely composed of silicon dioxide by subjecting them to aftertreatment with a number of agents. Some of these processes have even resulted in improved stability at high temperatures. Thus, German Patent Application No. 29 25 447 discloses a method in which glass fibers are treated, before or after their extraction, with silica, as polysilicic acid in colloidal or suspension form, or with dissolved silica salts to raise the SiO.sub.2 content of the glass fibers. This is done to increase tensile and buckling strength. Such a process also improves fiber resistance to high temperatures. But it does not produce fibers that maintain good characteristics at 1500.degree. C.
U.S. Pat. No. 4,362,768 discloses a method in which glass fibers are treated with a special leaching agent, namely tetrahydrofuran - 2,3,4,5 - tetracarboxylic acid. After drying, these fibers are treated with colloidal silica and then subjected to additional processing with an aqueous solution containing salts of metals such as chromium, aluminum, zirconium, titanium and metals in Group II of the periodic table. The fibers subjected to this aftertreatment are dried; they keep their flexibility even after treatment at temperatures as high as 871.degree. C. But this U.S. Patent does not provide any indication regarding the production of fibers that maintain very good stability at very high temperatures, i.e. for applications at 1500.degree. C. These aftertreatments, such as the use of chromium compound in particular, serve to maintain the glass characteristics of the fibers and to prevent crystallization.
German Patent Apecification No. 20 41 321 discloses a method for the production of silicon dioxide fibers involving the spinning of hydrolyzed tetra-alkoxy hydrosilicons or hydrolyzed alkoxy polysiloxanes in the presence of dissolved polyethylene oxide; as disclosed in this German specification, these fibers maintain their amorphous structure, even when heat treatments at temperatures of up to 1500.degree. C. are applied. German Patent Application No. 26 09 419 also describes silicon dioxide containing fibers that offer a very high degree of resistance to devitrification, i.e. that remain essentially amorphous. As the above documents demonstrate, the formation of cristobalite has, up to now, been considered undesirable.
Moreover, journals and the patent literature make numerous references to aftertreatment of inorganic fibers by a great variety of agents and describe fibrous materials in which the fibers are held together by a binding agent. German Patent Specification No. 24 46 278, for example, describes fiber compounds that contain a binding agent based on colloidal silicon dioxide and a siloxane-based matrix. The fiber compounds described therein may also contain cellulose fibers, so that the use of these fiber compounds at high temperatures is out of the question.
U.K. Patent Application No. 2,047,297 describes fibrous materials that are based on inorganic staple fibers, preferably of polycrystalline structure, such as alumina fibers, aluminosilicate fibers, zirconia fibers, iron oxide fibers or metal fibers. Phosphoric acid, phosphates, silica sols or aluminum chlorohydrate serve as binding agents. But no indications can be found in this patent application about the production, starting from amorphous silicon dioxide fibers, of fibrous material resistant to high temperatures.
In the known state of the art, polycrystalline fibers, consisting at least in large part of aluminum oxide, must be used to obtain objects that maintain dimensional stability at high temperature. Thus, Example 9 of U.K. Patent Application No. 2,047,297, cited above, describes pads that are made of polycrystalline fibers containing 95% alumina fibers (Saffil) and that maintain dimensional stability at 1400.degree. C. But the alumina fibers required to produce these insulation materials are expensive. Thus attempts have been made to replace such polycrystalline fibers entirely or in part by so-called ceramic fibers, a designation applied to amorphous fibers produced from the melt, comprising essentially, for instance, silicon dioxide and alumina in about equal parts. Although this method produces significantly cheaper insulation materials, the trade-off is reduced resistance to high temperatures.
While many aftertreatment methods for inorganic fibers are already well known, and it is also known how to produce materials such as fiber mats, plates and other objects from such fibers, there remains the need for a high temperature-resistant fibrous material that is easy and cost-efficient to manufacture, is stable at high temperatures, especially at about 1500.degree. C. and above, undergoes minor shrinkage and has little tendency to become brittle.